Servo motors

Can anybody give me some links to possible sources for servo motors suitable for use with smaller CNC machines. Apart from the 'refer to sales office' links the best I've come up with so far is a site offering motors at £1200 up to £3500. Somehow that seems a little excessive when the mill only cost £650.
I know we will be looking at a price premium over the closed loop stepper option using 0.2Nm motors, but there must be something suitable to use with the Gecko servo drive at a reasonable price? I presume I am looking for something ideally with a Nema23 mount and I think 0.4Nm seems practical in that form factor? But with an assembly somewhat longer than the 0.2Nm stepper with encoder?

wrote:
  Quoted Text Here  
Lester -
Are you sure that you are talking about 0.2 Nm?? That's approximately sod all - or 28 oz-in in real money.
Regards, Tony

Tony Jeffree wrote:
  Quoted Text Here  
You are right - I'm trying to compare different units and getting it wrong :)
I'm looking at the MAC drives http://www.jvl.dk/files/pdf/Ld043gb.pdf But it would seem that I need a gear box to get the same power as the equivalent stepper? I understood servos had a lot more power? Where am I going wrong ....

wrote:
  Quoted Text Here  
You're going wrong by confusing power and torque.
Power is torque times RPM.
These (servo) motors are high revving (4000 RPM is very fast indeed for a stepper), and develop their highest power output at high RPM; by the same token, they generate relatively little torque. So yes, to get sensible performance out of them when driving a small mill, both in terms of generating sufficient torque and in terms of keeping the slew rates within bounds, you would need to gear them down (probably by a factor of around 4:1 I suspect in this case). In doing so you will up the available torque by the gearing ratio (less any frictional losses in the gearing).
Regards, Tony

Tony Jeffree wrote:
  Quoted Text Here  
I think I had just about got there ... Bottom line - I don't see any advantage in trying to add servo's to a Taig size mill when compared with the closed loop stepper motor set-up. But I'm waiting on prices from a couple of sources at the moment anyway and given the longer motor length so needing to wrap the motor around the back using a timing belt, 5 to 1 reduction would be practical anyway.

  Quoted Text Here  
Just being a little bit picky here:-
A closed loop stepper motor setup would be quite rare and probably only be used by people called Gareth :-0
Everyone else would use them open loop.
Mark Rand RTFM

On Mon, 12 May 2008 21:35:03 +0100, Mark Rand
  Quoted Text Here  
Mark -
Actually, Taig offer a closed loop stepper setup.
Regards, Tony

  Quoted Text Here  
Now when I asked about closed loop set ups using steppers, the response was that steppers are upto it and you had to use servos to implement closed loop...
So much for "Power of the crowd"!
Steve

Steve wrote:
  Quoted Text Here  
I am supplying currently both open loop and closed loop stepper motor systems using the Taig/Microproto controller. The closed loop give a three fold improvement in speed, but since it is implemented in the controller and only gives feedback to Mach3 when there is an error condition, I suspect Mach3 is getting ahead of the game from time to time. So current plan is an improved link with Mach3. Brian at Newfangled is due to send me a plugin for Mach3 which will run this improved arrangement.
The reason for looking at servo motors was to compare prices, but I am finding that at this size of motor the stepper does seem to be preferable? Once you can monitor it's position ;)
Now the next step is perhaps glass scales on the bed in place of the encoders direct to the motor?

Lester Caine wrote:
  Quoted Text Here  
Oh well - from £400 for a similar sized servo motor and I'm not sure if that includes in integral drive as against £75 for the stepper motor option complete - per channel ...

wrote:
  Quoted Text Here  
I hate to suggest it but...
http://www.cadcamcadcam.com/index.asp?PageAction=VIEWPROD&ProdIDb
Regards, Tony

Lester Caine wrote:
  Quoted Text Here  
Lester. Take a look at the Italian motors supplied by motion control products in Bournemouth. For small motors they can be quite competitive. I use some 300w motors of this type on my Hardinge.
It's not really useful to compare steppers holding torque against a servos peak torque. In my experience with larger machinery a servo with much less peak torque than a steppers holding torque will have the stepper for breakfast, dinner and tea!
MCP also supply the leadshine dc servo drive which is no longer a gecko copy. It's better in fact.
Wayne.....

On Mon, 12 May 2008 22:48:19 +0100, "Steve"
  Quoted Text Here  
See:
http://en.wikipedia.org/wiki/Servo
Quote: "Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback."
So the Taig system does indeed use servos. But with stepper motors.
Next question?
Regards, Tony

  Quoted Text Here  
There has also been mention of a Servo Stepper driver on the Gecko forum at cnczone. I think a rough time scale for availability is later this year.

  Quoted Text Here  
That's sort of right conceptually, but just in case you are tempted to actually try to calculate a 'real' power
Power (W) = torque (Nm) * omega (speed in rads/sec or RPM*Pi/30)
Richard

On Mon, 12 May 2008 17:58:04 +0100, Richard
  Quoted Text Here  
...So I didn't say what the units were <G>
Regards, Tony

  Quoted Text Here  
I have 2 motors stuck under the bench at work, I'll have a look and send the details from the label. Bob